1. Field of the Invention
The present invention relates to a container having a reduced amount of acetaldehyde released therefrom and a method of molding that container. Particularly, the present invention relates to a method of molding a food packaging container of polyester resin having a reduced amount of acetaldehyde released therefrom, particularly a non-heat-resistant container, and also relates to such a non-heat-resistant container itself.
2. Prior Art
There are broadly known containers for mineral water, carbonated beverages, or the like, which are formed by biaxial stretch blow molding of a saturated polyester such as polyethylene terephthalate. Such containers are called xe2x80x9cPET bottlesxe2x80x9d.
It is also known that the polyester resin used to form such containers includes acetaldehyde. Acetaldehyde may be newly produced from the thermal decomposition of polyester due to residence or shear heating during the injection molding or extruding process, in addition to acetaldehyde included in the original polyester material.
Acetaldehyde is an organic substance having a peculiar smell and raises a problem in that it affects the taste of a beverage contained in a container when polyester resin is used to form the container.
It is therefore desired to reduce the amount of acetaldehyde released from the containers to as little as possible. Many proposals for reduction of acetaldehyde have been made heretofore.
Almost all the proposals relate to reduction of acetaldehyde in the polyester materials. Some of them are commercially successful, but resultant materials are very expensive in comparison with the conventional materials, leading to increase of cost in the product.
Even if such very expensive materials are used, one could not greatly reduce the amount of acetaldehyde produced on the injection molding or other process.
The applicant has proposed some methods of making heat-resistant containers as described in Japanese Patent Publication Nos. Hei 6-88314, Hei 8-5118 and Hei 6-55418.
In these proposals, a preform is stretch-blow-molded into a primary blow-molded article in the form of a container and then it is heated in an oven to improve its criplallinity and to be shrunk by heat before it is stretch-blow-molded into a final product or a heat resistant container.
Particularly, the methods of Japanese Patent Publication Nos. Hei 6-88314 and Hei 8-5118 are epochal in that acetaldehyde can greatly be reduced by heat.
In the just mentioned methods, however, the blow-molded article is heated by passing it through a long oven for a relatively long time between 20 and 120 seconds and preferably no shorter than 30 seconds.
Such a relatively long heating time lengthens the molding cycle, decreases the productivity per unit time and increases the running cost. By the use of an oven, the entire length of the molding machine is elongated to increase the cost of equipment and maintenance. As a result, the unit price of containers will increase.
It is acceptable that the heat-resistant containers are higher in cost than non-heat-resistant containers since the heat-resistant containers have an added value of heat resistance. However, the non-heat-resistant containers must be reduced in cost.
Therefore, the reduction of acetaldehyde in the non-heat-resistant containers only now depends on selection of the molding material and injecting condition. This means the reduction of acetaldehyde is limited.
It is therefore an object of the present invention to provide a method of molding non-heat-resistant containers which purposely adopts a process of molding heat-resistant containers and which can reduce the amount of acetaldehyde released from the molded containers while greatly reducing the heating time in comparison with the heat-resistant container molding process, and to provide a container molded by such a method.
Another object of the present invention is to provide a method of efficiently molding a container having a reduced amount of acetaldehyde released therefrom by thinning the wall thickness of the primary blow-molded article to shorten the heating time without increase of the entire system and without reduction of the productivity, and to provide a container molded by such a method.
Still another object of the present invention is to further shorten the heating time by the contact heating of a primary blow-molded article to improve the molding efficiency.
According to a first aspect of the present invention, there is provided a container molding method comprising:
a primary blow molding step of stretch-blow-molding a preform into a primary blow-molded article;
an acetaldehyde reducing step of reducing the amount of acetaldehyde released from the primary blow-molded article; and
a final blow molding step of stretch-blow-molding the primary blow-molded article having a reduced amount of acetaldehyde into a final container;
wherein the primary blow molding step molds the primary blow-molded article which is larger in size than the final container and which has an average wall-thickness of a stretched barrel equal to or less than 0.3 mm, that is smaller than an average wall-thickness of a barrel of the final container,
wherein the acetaldehyde reducing step includes a step of heating the thin-walled barrel of the primary blow-molded article at a high temperature,
whereby the amount of acetaldehyde released from an inner wall of the final container is reduced.
A container having an extremely reduced amount of acetaldehyde released or appearing therefrom can be molded by stretch-blow-molding a preform made of a conventional resin material or a resin material having a lower amount of acetaldehyde into a primary blow-molded article having a thin-walled barrel, and then heat treating the primary blow-molded article before the primary blow-molded article is blow molded into the final container.
Particularly, since the primary blow-molded article is larger than the final container with the average wall-thickness of its barrel being equal to or smaller than 0.3 mm, that is thinner than that of the final container, the heating step in the acetaldehyde reducing step can be carried out for a shortened time to improve the productivity. In addition, the amount of acetaldehyde released from the inner wall of the final container can be efficiently reduced. The reduction of heating time enables the installation space to be reduced, leading to reduction of the system size and manufacturing cost.
Since the primary blow-molded article is heated to reduce the amount of acetaldehyde released from the primary blow-molded article before the final blow molding step, the precision of configuration in the final blow molding step will be improved.
The resultant container is good for use as a non-heat-resistant container which is to be filled with a content at a temperature of 65xc2x0 C. or less and particularly with a mineral water or other beverage. This is because the smell peculiar in acetaldehyde is greatly reduced therein.
The average wall-thickness in the barrel of the final container obtained by stretch-blow-molding the primary blow-molded article which has its average wall-thickness of 0.3 mm or less in the barrel thereof is less than 0.3 mm.
When the average wall-thickness in the barrel of the final container is less than 0.3 mm, the average wall-thickness in the barrel of the primary blow-molded article becomes extremely thin. Thus time required to reduce the amount of acetaldehyde can be shortened to improve the efficiency in molding.
The acetaldehyde reducing step may include a step of heating the primary blow-molded article at a temperature of 140xc2x0 C. or higher. This is because the amount of acetaldehyde released from the inner wall of the final container is sharply reduced at a temperature equal to or higher than 140xc2x0 C. In such a case, the heating time for the primary blow-molded article can be shortened to six seconds or less. If the heating temperature is equal to or higher than 190xc2x0 C., the heating time will be equal to or less than one second. In such a manner, the high reduction of acetaldehyde released from the inner wall of the final container can be accomplished for an extremely short time, resulting in more improvement of the productivity.
The acetaldehyde reducing step may heat the primary blow-molded article so that the amount of acetaldehyde released from per unit surface area of the primary blow-molded article will be equal to or less than 0.0040 pg/cm2. This can reduce the amount of acetaldehyde released from per unit surface area of the final container to a level equal to or less than 0.0020 xcexcg/cm2 when the final container is stored for 24 hours at 22xc2x0 C. after the final blow molding step.
The acetaldehyde reducing step can make the primary blow-molded article in contact with an inner wall of a mold for heating, when this mold is substantially the same size as the primary blow-molded article and has been heated. When the thin-walled barrel of the primary blow-molded article is heated in contact with the heated inner mold wall of the mold which has substantially the same size as the primary blow-molded article, the heating time can be shortened to improve the productivity. Further, since the space for the heating step may be also the space for the mold, the blow molding machine can be reduced in size with reduced cost.
According to a second aspect of the present invention, there is provided a container producing method comprising:
a primary blow molding step of stretch-blow-molding a preform into a primary blow-molded article;
an acetaldehyde reducing step of reducing the amount of acetaldehyde released from the primary blow-molded article; and
a final blow molding step of stretch-blow-molding the primary blow-molded article having the reduced amount of acetaldehyde into a final container,
wherein the primary blow molding step molds the primary blow-molded article so that a ratio of weight to surface area becomes equal to or less than 0.06 g/cm2,
wherein the acetaldehyde reducing step heats a barrel of the primary blow-molded article at a high temperature,
whereby the amount of acetaldehyde released from an inner wall of the final container is reduced.
If the ratio of weight to the surface area becomes equal to or less than 0.06 g/cm2, the average wall-thickness in the barrel of the primary blow-molded article becomes extremely thin. Thus, the heating in the acetaldehyde reducing step can be carried out for a reduced time, resulting in improvement of the productivity.